1. Field of the Invention
The present invention relates generally to an encoding/decoding apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for transmitting a reverse rate indicator channel (R-RICH) used in a synchronous mobile communication system.
2. Description of the Related Art
In general, a reverse supplemental channel (R-SCH) fundamentally supports a variable rate transmission scheme. In the “variable rate transmission scheme”, a mobile station varies its transmission rate arbitrarily. Generally, a change in a data rate causes a change in a code rate of error correcting codes used in constructing a frame, a symbol repetition frequency, and a length and type of Walsh codes for spreading. Therefore, the mobile station should inform a base station of a data rate of the currently transmitted reverse supplemental channel, so that a base station receiver can correctly receive the reverse supplemental channel. A channel defined for this use is called a reverse rate indicator channel (R-RICH).
The number of available data rates at which the mobile station can transmit data over the reverse supplemental channel depends upon the number of available reverse supplemental channels that the mobile station can use at the same time. The number of the reverse supplemental channels is determined by the base station considering an amount of data to be transmitted over a reverse link during call setup, and then reported to the mobile station. Therefore, the number of information bits transmitted over the reverse rate indictor channel varies depending on the number of the reverse supplemental channels. That is, when the number of the reverse supplemental channels is 1, the mobile station informs a reverse data rate using 4 bits. Further, when the number of the reverse supplemental channels is 2, the mobile station informs the reverse data rate using 7 bits. Since the number of the available reverse supplemental channels that can be simultaneously used by the mobile station cannot be changed until a separate command is received from the base station, the mobile station transmits either 4-bit information or 7-bit information over the reverse rate indicator channel. That is, the mobile station will never transmit both the 4-bit information and the 7-bit information. Conventionally, a (24,4) or a (24,7) code is defined as an error correcting code to be used in the reverse rate indicator channel.
FIG. 1 illustrates a structure of an R-RICH transmitter. Referring to FIG. 1, an encoder 100 encodes a 4-bit or 7-bit input rate indicator and outputs 24 coded symbols. A symbol repeater 110 repeats 16 times the 24 coded symbols provided from the encoder 100. A signal mapper 120 performs signal mapping on the coded symbols output from the symbol repeater 110 by mapping 0 to 1 and 1 to −1. A spreader 130 spreads the signal-mapped symbols.
As illustrated in FIG. 1, the rate indicator having 4 bits or 7 bits, and encoded into 24 coded symbols before transmission. When errors occur during transmission of the rate indicator encoded into the coded symbols, the rate indicator may incorrectly indicate the code rate of the corresponding reverse supplemental channel, the symbol repetition frequency, and the length and type of the spread Walsh code. As a result, the receiver cannot correctly analyze the reverse supplemental channel. Therefore, the rate indicator should be encoded by a (24,4) or a (24,7) encoder having good performance. In addition, the rate indicator should be decoded as fast as possible in order to analyze the corresponding supplemental channel.